In metal electroplating upon a non-planar substrate, it has generally been recognized to be advantageous to employ a simple elongate or rod-like electrode which is slender and straight and may generally be continuous in composition over its length (hereinafter "rod electrode" or "electrode rod"). The rod electrode is energized in an electroplating system in which it is poled as anodic relative to a workpiece of a larger area poled cathodic and is, in the presence of a plating electrolyte, juxtaposed with the latter to provide an active plating electrode face at its end portion to allow electrodeposition of a metal from the electrolyte upon the region of the workpiece or substrate surface proximal to the active electrode face. In the process, the rod electrode is displaced in a scanning manner along the surface of the substrate to successively and continuously electrodeposit the metal over the entire area of the substrate.
In practicing the electrode scanning operation, it is conceivable to control both the position and attitude of the rod electrode such that its axis is oriented always perpendicular to the tangent to the substrate surface at any point and its end portion constituting the active anode face is always spaced with a fixed distance from the substrate surface at any point while the residence time of the electrode active face relative to each point on the surface is kept to be uniform. In this case the greater the interelectrode distance, the greater is the loss in consumed electric power and the more irregular becomes the plated layer in thickness. On the other hand, the reduction of the distance forced the electrode to move along an increased path of scanning, entailing a greater length of displacement program. An excessive undulation in scanning may also ensue. It is therefore desirable that a proper selection be made for the dimension of the interelectrode distance to be maintained during the scanning operation in accordance with the particular configuration and area of the substrate to be electroplated. To allow the attitude control of the rod electrode from one position to another, however, necessitates a complicated and costly arrangement and also entails complicated calculations in the preparation of a suitable scanning operation program, and cannot therefore be practical.
Accordingly, in the procedure followed heretofore, the rod electrode is retained with its axis always oriented parallel with a given axis (z-axis) or perpendicular to a given plane (x-y plane) and is displaced so that its active end moves along an imaginary curvature defined by the parallel shift, along the same axis, of the surface contour of the substrate by a distance of interest. In this procedure, the latter distance or the distance measured from the electrode end along the extension of the electrode axis to the point of its intersection with the surface contour or the substrate is held to be constant. This procedure allows preparation of a necessary scanning path program with ease and poses no material problem where the substrate contains no portion of acute inclination on its surface with respect to the extension of the electrode axis. When, however, such portion exists, the minimum distance between the active electrode face and the substrate is substantially reduced there, tending to cause an arc discharge or short-circuiting which is detrimental. Furthermore, electrodeposition tends to build up excessively and the active electrode face may erode irregularly in such areas. As a consequence, the operation becomes unsatisfactory both with respect to performance and plating precision.